Monitoring

Anteater
A log analyser for MTA logfiles (for example sendmail). (C++) [GNU/Linux, Unix]
http://anteater.drzoom.ch/

AWStats
A web server logfile analyzer that works as a CGI and/or from command line, and supports multiple languages. (Perl)
http://awstats.sourceforge.net/

Calamaris
Parses the logfiles of a wide variety of Web proxy servers and generates reports. (Perl) [Linux, Unix]
http://cord.de/tools/squid/calamaris/Welcome.html

CoCaLoRes
Resolves IP numbers to hostnames like in Apache, Squid, or other logfiles. It is able to cache lookups in a file. (Perl)
http://cocalores.sourceforge.net/

Fast Track Systems Monitor
A general purpose monitoring engine for use by home users and system administrators (Perl) [Unix, Win32]
http://ftmon.sourceforge.net

Fireparse
Emails a report of all packets that have been logged by the kernel's ipchains or iptables packet filtering subsystems. (Perl) [GNU/Linux]
http://freshmeat.net/projects/fireparse/

Htmlobserver
Web page checker based on regular expressions. (Java) [Linux, Windows]
http://home.arcor.de/hirnstrom/htmlobserver/index.html

Iperf
A tool to measure the maximum TCP bandwidth between two points on the internet. (C++) [Many platforms]
http://dast.nlanr.net/Projects/Iperf/

Netload-Applet
A network load monitoring applet for GNOME. (C) [GNU/Linux, FreeBSD]
http://netload-applet.sourceforge.net/

Network Grep
Ngrep is a pcap-aware tool that will allow you to specify extended regular expressions to match against data part of packets on the network. (C) [GNU/Linux, BSD, Windows 95/98/2000]
http://ngrep.sourceforge.net/

NISCA
A network interface traffic statistics reporting and graphing tool. (PHP)
http://nisca.sourceforge.net/

Ntop
A tool that shows the network usage. It can be used interactively, or in web mode. (C) [GNU/Linux, Unix, Windows]
http://www.ntop.org/ntop.html

Osmius
Allows the monitoring of heterogeneous systems using multi-platform agents distributed over a network. Includes news, downloads and a development weblog.
http://www.osmius.net

RackView
Assists in visualizing a Data Center's computer rack layouts by creating a web page with HTML tables to show a frontview of the racks. Included are brief commandline and CGI scripts to demonstrate its implementation. (Perl) [Linux]
http://rackview.sourceforge.net

squidefender
Perl script which parses for attacks a squid log file. Allows to run commands or automatically notify the attacker's ISP upon trigger.
http://www.jeroen.se/squidefender.php

Zabbix
Software for application and network monitoring - logging, monitoring, capacity planning, availability and performance measurement, and helpdesk information. (C, PHP) [POSIX]
http://www.zabbix.com/